1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for positioning a mobile station within a cellular network, and specifically to positioning a mobile station using a time of arrival (TOA)-based positioning method.
2. Background of the Present Invention Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. A standardization group, European Telecommunications Standards Institute (ETSI), was established in 1982 to formulate the specifications for the Global System for Mobile Communication (GSM) digital mobile cellular radio system.
With reference now to FIG. 1 of the drawings, there is illustrated a GSM Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC 14 provides a circuit switched connection of speech and signaling information between a Mobile Station (MS) 20 and the PLMN 10. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which the MS 20 may move freely without having to send update location information to the MSC 14 that controls the LA 18. Each LA 18 is divided into a number of cells 22. The MS 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless.
The MSC 14 is in communication with at least one Base Station Controller (BSC) 23, which, in turn, is in contact with at least one Base Transceiver Station (BTS) 24. The BTS is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the cell 22 for which it is responsible. It should be understood that the BSC 23 may be connected to several BTS""s 24, and may be implemented as a stand-alone node or integrated with the MSC 14. In either event, the BSC 23 and BTS 24 components, as a whole, are generally referred to as a Base Station System (BSS) 25.
The MS 20 and the BTS 24 communicate over a radio interface, which utilizes the Time Division Multiple Access (TDMA) concept. Each TDMA frame consists of a number of time slots, with one time slot per carrier frequency. Each time slot is referred to as a physical channel. Depending upon the type of information being transmitted, different types of logical channels are mapped onto these physical channels. For example, to transmit speech, the logical channel xe2x80x9ctraffic channelxe2x80x9d must be mapped onto one of the physical channels. The information sent on one of these channels is called a burst. In addition, the TDMA frames are numbered in a cyclic pattern.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information, for subscribers registered within that PLMN 10. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
Determining the geographical position of an MS 20 within a cellular network 10 has recently become important for a wide range of applications. For example, location services (LCS) may be used by transport and taxi companies to determine the location of their vehicles. In addition, for emergency calls, e.g., 911 calls, the exact location of the MS 20 may be extremely important to the outcome of the emergency situation. Furthermore, LCS can be used to determine the location of a stolen car, for the detection of home zone calls, which are charged at a lower rate, for the detection of hot spots for micro cells, or for the subscriber to determine, for example, the nearest gas station, restaurant, or hospital, e.g., xe2x80x9cWhere am Ixe2x80x9d service.
As can be seen in FIG. 2 of the drawings, upon the reception of a positioning request from a Location Services (LCS) client 280, the MSC 14 sends a Mobile Application Part (MAP) PERFORM LOCATION message to a Serving Mobile Location Center (SMLC) 270 within the PLMN 10 associated with the MSC 14. The SMLC 270 is responsible for carrying out the positioning request and calculating the MS 20 location. It should be noted that more than one SMLC 270 may be located within each PLMN 10. Thereafter, the SMLC 270 determines the positioning method to use. If the Time of Arrival (TOA) positioning method is selected, the SMLC 270 returns a MAP CHANNEL INFORMATION message to the MSC 14. The MSC 14, in turn, forwards a Base Station Subsystem MAP (BSSMAP) CHANNEL INFORMATION message to the serving BSC 23, requesting the physical channel description of the traffic channel that will be used to perform a positioning handover. The message also includes information on the cell 22 ID""s and TDMA frame numbers for the serving and candidate cells 22 to which positioning handovers are to be performed.
In response, the BSC 23 sends a BSSMAP CHANNEL INFORMATION ACK message to the MSC 14, which includes the requested physical channel description. The MSC 14 forwards this physical channel description to the SMLC 270, which uses the physical channel description to configure at least three Location Measurement Units (LMUs) 260 (only one of which is shown) within the PLMN 10. The LMUs 260 are responsible for obtaining positioning measurements and providing these measurements to the SMLC 270 for use in calculating the location of the MS 20. All communication to and from the LMUs 260 are sent over the air interface. Therefore, unless the LMU 260 is integrated with a BTS 24, each LMU 260 is in wireless communication with at least one associated BTS 24.
Once the SMLC 270 selects which LMUs 260 should obtain the positioning measurements, the SMLC 270 sends LCS Information Request messages to each of these selected LMUs 260. The LCS Information Request messages specify the absolute time that the LMUs 260 should begin to measure the time of arrival (TOA) of access bursts transmitted by the MS 20. The LMUs 260 only have knowledge of the absolute time, and do not have any information on the current TDMA frame number. Therefore, this absolute time must correspond to the TDMA frame number transmitted by the SMLC 270 to the BSC 23 in the CHANNEL INFORMATION message in order for positioning to occur. For example, if the transmitted TDMA frame number corresponds to starting time t0 for one of the LMUs 260, but the SMLC 270 instructs that LMU 260 to begin listening at starting time t1, then that LMU 260 will not begin listening to the access bursts transmitted by the MS 20 at the correct time. Thus, that LMU 260 will not be able to obtain positioning measurements, and, as a consequence, the SMLC 270 will not be able to calculate the location of the MS 20.
Therefore, TOA-based positioning requires knowledge of the relation between the absolute time and air interface timing, e.g., the TDMA frame number. Currently, the LMUs 260 monitor the TDMA frame number on a synchronization channel of an associated BTS 24 and determine the corresponding absolute time using a GPS receiver within the LMU 260 for that TDMA frame number. Thereafter, the LMU 260 periodically sends this information to the SMLC 270, which, in turn, stores this relation information in a database 275 therein. Later, when a positioning request is received, the SMLC 270 uses this relation information in the LMU 260 configuration process.
The periodicity of transmission of this relation information from the LMU 260 to the SMLC 270 can be as often as every 2 to 5 minutes. This creates increased signaling load on the network, especially when the SMLC 270 has not received a positioning request for that cell 22. Thus, the LMUs 260 are making these measurements and sending the results to the SMLC 270 even when the SMLC 270 does not need this information.
The present invention is directed to telecommunications systems and methods for reducing the signaling load on the network by instructing a Location Measurement Unit (LMU) to measure the relation between the absolute time and air interface timing, e.g., TDMA frame number, only when a positioning request has been received. Prior to configuring the LMU for time of arrival (TOA)-based positioning of a particular mobile station, the Serving Mobile Location Center (SMLC) can request an LMU associated with a particular cell to perform an asynchronous handover within the same cell. During this asynchronous handover, the LMU measures the relation between the absolute time and the TDMA frame number, and sends this relation information to the SMLC. The SMLC uses this relation information in configuring the LMU for TOA-based positioning of the particular mobile station.